This invention relates generally to rotation sensors and particularly to ring laser gyroscope rotation sensors. Still more particularly, this invention relates to apparatus and methods for stabilizing the frequency of light produced in a ring laser.
A ring laser gyroscope employs the Sagnac effect to detect rotation. Two counterpropagating light beams in a closed loop will have transit times that differ in direct proportion to the rotation rate of the loop about an axis perpendicular to the plane of the loop. There are in general two basic techniques for utilizing the Sagnac effect to detect rotations. A first technique is the interferometric approach, which involves measuring the differential phase shift between two counterpropagating beams injected from an external source, typically a laser, into a Sagnac ring. The ring may be defined by mirrors that direct the light beams around the path or by a coil of optical fiber. Beams exiting the path interfere and create a pattern of light and dark lines that is usually called a fringe pattern. Absolute changes in the fringe pattern are indicative of rotation of the ring. The primary difficulty with such devices is that the changes are very small for rotation rates of interest in guidance applications.
The ring laser gyroscope uses the resonant properties of a closed cavity to convert the Sagnac phase difference between the counter propagating beams into a frequency difference. The high optical frequencies of about 10.sup.15 Hz for light used in ring laser gyroscopes cause the minute phase changes to become beat frequencies that are readily measured. The cavity length must be precisely controlled to provide stability in the lasing frequency. A stable frequency is required to provide the desired accuracy in measuring rotations.
A ring laser gyroscope has a sensor axis that passes through the closed paths traversed by the counterpropagating beams. When the ring laser gyroscope is not rotating about its sensor axis, the optical paths for the two counterpropagating beams have identical lengths so that the two beams have identical frequencies. Rotation of the ring laser gyroscope about its sensor axis causes the effective path length for light traveling in the direction of rotation to increase while the effective path length for the wave traveling in the direction opposite to the rotation decreases.
Ring laser gyroscopes may be classified as passive or active, depending upon whether the lasing, or gain medium is external or internal to the cavity. In the active ring laser gyroscope the cavity defined by the closed optical path becomes an oscillator, and output beams from the two directions beat together to give a beat frequency that is a measure of the rotation rate. The oscillator approach means that the frequency filtering properties of the cavity resonator are narrowed by many orders of magnitude below the passive cavity and give very precise rotation sensing potential. To date the major ring laser gyroscope rotation sensor effort has been put into the active ring laser. Presently all commercially available optical rotation sensors are active ring laser gyroscopes.
When the rotation rate of the ring laser gyroscope is within a certain range, the frequency difference between the beams disappears. This phenomenon is called frequency lock-in, or mode locking, and is a major difficulty with the ring laser gyroscope because at low rotation rates, the frequency difference between the beams disappears. This input rotation rate is called the lock-in threshold. The range of rotation rates over which lock-in occurs is the deadband of the ring laser gyroscope.
Lock-in is believed to arise from coupling of light between the beams. The coupling results primarily from backscatter off the mirrors that confine the beams to the closed path. Backscatter causes the beam in each direction to include a small component having the frequency of the beam propagating in the other direction. The lock-in effect in a ring laser gyroscope is similar to the coupling that has long been observed and understood in conventional electronic oscillators.
Upon reversal of the sign of the frequency difference between the two beams, there is a tendency for the beams to lock-in since at some point the frequency difference is zero. Since the output of the ring laser gyroscope is derived from the frequency difference, an error accumulates in the output angle. The periods in which the two beams are locked in are usually very short in duration; thus, the error is very small. However, since the error is cumulative, in time the error can become appreciable in precision navigation systems. This error is a major contributor to random walk or random drift.
In addition to causing erroneous rotation rate information to be output from a ring laser gyroscope, lock-in causes standing waves to appear on the mirror surfaces. These standing waves may create a grating of high and low absorption regions, which create localized losses that increase the coupling between the beams and the lock-in. The mirrors may be permanently distorted by leaving a ring laser gyroscope operating in a lock-in condition.
Any inability to accurately measure low rotation rates reduces the effectiveness of a ring laser gyroscope in navigational systems. There has been substantial amount of research and development work to reduce or eliminate the effects of lock-in to enhance the effective use of ring laser gyroscopes in such systems.
There are several known approaches to solving the problems of lock-in. One such approach involves mechanically oscillating the ring laser gyroscope about its sensor axis so that the device is constantly sweeping through the deadband and is never locked therein. This mechanical oscillation of the ring laser gyroscope is usually called dithering.
In one implementation of the RLG, lock-in is largely subdued by placing a sinusoidal plus random mechanical rotational dither of about 200 arc seconds amplitude and at a rate of a few hundred Hertz on the input axis of the RLG. This greatly reduces the scale factor nonlinearity but does not completely eliminate it.. The present invention seeks to minimize scale factor variations and nonlinearities by controlling the scatter and by providing a discriminant for correction of scale factor error.
Basically, there are two mirror scatterer types: that due to surface roughness or height variation, H, which induces a common mode phase shift of 90 degrees, .pi./2 radians, and that due to dielectric variations, D, which induces a phase shift of 0 degrees. These 2 scattering sources also have asymmetrical phase shifts which result in the clockwise (cw) traveling scattered field appearing as the complex conjugate of the counter-clockwise (ccw) scattered field.
Previous methods of scale factor variation reduction involved reducing scatter and increasing dither amplitude. These have distinct limitations in terms of cost and mechanical complexity. The present invention attempts to reduce scale factor nonlinearity by an ensemble average of a factor of 4 and may reduce the variations by a much larger factor, all for a moderate cost and moderate increase in complexity.